Group16_QuadMasters:Selective Repeat ARQ ppt

Group16_QuadMasters:Selective Repeat ARQ

Features required for Selective Repeat ARQ

* To support Go-Back-N ARQ, a protocol must number each PDU which is sent. (PDUs are normally numbered using modulo arithmetic, which allows the same number to be re-used after a suitably long period of time. The time period is selected to ensure the same PDU number is never used again for a different PDU, until the first PDU has “left the network” (e.g. it may have been acknowledged)).
* The local node must also keep a buffer of all PDUs which have been sent, but have not yet been acknowledged.
* The receiver at the remote node keeps a record of the highest numbered PDU which has been correctly received. This number corresponds to the last acknowledgement PDU which it may have sent.

The above features are also required for Go-Back-N, however for selective repeat, the receiver must also maintain a buffer of frames which have been received, but not acknowledged.

Group16_QuadMasters:Selective Repeat ARQ

Applications of Selective Repeat ARQ

• TCP (Transmission Control Protocol):  transport layer protocol uses variation of selective repeat to provide reliable stream service
• Service Specific Connection Oriented Protocol:  error control for signaling messages in ATM network

Efficiency of Selective Repeat

• Assume Pf frame loss probability, then number of transmissions required to deliver a frame is:

• tf / (1-Pf)

Example:  Impact Bit Error Rate on Selective Repeat

nf=1250 bytes = 10000 bits, na=no=25 bytes = 200 bits

Compare S&W, GBN & SR efficiency for random bit errors with p=0, 10-6, 10-5, 10-4 and  R= 1 Mbps & 100 ms

Efficiency	0	10-6	10-5	10-4
S&W	8.9%	8.8%	8.0%	3.3%
GBN	98%	88.2%	45.4%	4.9%
SR	98%	97%	89%	36%

• Selective Repeat outperforms GBN and S&W, but efficiency drops as error rate increases

Group16_QuadMasters:Selective Repeat ARQ

Selective Repeat ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol. It may be utilized as a protocol for the delivery and acknowledgement of message units, or it may be utilized as a protocol for the delivery of subdivided message sub-units.

Performance of Selective Repeat ARQ :

Efficiency of Selective Repeat ARQ:

It is the most efficient of all the ARQ protocols

that have been discussed.

The probability of a frame being received in good order is

1 – Pf

Thus the average number of times a frame has to be sent

is the reciprocal of the probability, i.e., 1/(1 – Pf)

the  average transmission is :      tSR  =Tf/ 1-Pf

When used as the protocol for the delivery of subdivided messages it works somewhat differently. In non-continuous channels where messages may be variable in length, standard ARQ or Hybrid ARQ protocols may treat the message as a single unit. Alternately selective retransmission may be employed in conjunction with the basic ARQ mechanism where the message is first subdivided into sub-blocks (typically of fixed length). The original variable length message is thus represented as a concatenation of a variable number of sub-blocks. While in standard ARQ the message as a whole is either acknowledged (ACKed) or negatively acknowledged (NAKed), in ARQ with selective transmission the NAKed response would additionally carry a bit flag indicating the identity of each sub-block successfully received. In ARQ with selective retransmission of sub-divided messages each retransmission diminishes in length, needing to only contain the sub-blocks that were NAKed.

In most channel models with variable length messages, the probability of error-free reception diminishes in inverse proportion with increasing message length. In other words it’s easier to receive a short message than a longer message. Therefore standard ARQ techniques involving variable length messages have increased difficulty delivering longer messages, as each repeat is the full length. Selective retransmission applied to variable length messages completely eliminates the difficulty in delivering longer messages, as successfully delivered sub-blocks are retained after each transmission, and the number of outstanding sub-blocks in following transmissions diminishes.

Group16_QuadMasters:Selective Repeat ARQ

Selective Repeat ARQ is a specific instance of the Automatic Repeat-request (ARQ) Protocol. It may be utilized as a protocol for the delivery and acknowledgment of message units, or it may be utilized as a protocol for the delivery of subdivided message sub-units. When used as the protocol for the delivery of messages, the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N ARQ, the receiving process will continue to accept and acknowledge frames sent after an initial error.

The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every ACK it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window. The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame. Once the sender has sent all the frames in its window, it re-sends the frame number given by the ACKs, and then continues where it left off. The size of the sending and receiving windows must be equal, and half the maximum sequence number (assuming that sequence numbers are numbered from 0 to n-1) to avoid miscommunication in all cases of packets being dropped. The sender moves its window for every packet that is acknowledged.

• Transmitter’s rule of action
1) Keeps transmitting frames continuously.
2) If any frame is not acknowledged Tout after the frame is transmitted (or receives NACK), assume that the frame is lost and retransmit only the lost frame.

• Receiver’s rule of action
1) Does the same action as in the Stop-and-Wait ARQ, but never discard all frames delivered without errors and has the capability to reorder them in the correct order.
2) Sequence numbers are required to identify different frames.

Group16_RemoteLogin:Interactions Blog-Group A Project Q & A Session

Q) Whether the Operating System should be the for Remote login to happen?

Ans: Need not be Remote login does not depend on the Operating system used

Anwered by Hariprasad

Q) IP will be changing dynamically so how will remote login happen with a single IP

Ans: For each session there will be different  ip addresses  and therefore remote login  in different sessions may have different ip addresses

Answered by Hariprasad

Q) Can we remote login using other than ip address in telnet?

Ans:Need not be ip address always we can also use  host name for remote login

Answered by Deepakraj

Q) What is VPN network?

Ans: A virtual private network (VPN) is a computer network that is implemented in an additional software layer (overlay) on top of an existing larger network for the purpose of creating a private scope of computer communications or providing a secure extension of a private network into an insecure network such as the Internet.

Answered by Pavan.S

Group16_QuadMasters:Remote Login in Windows

Group16_QuadMasters:Remote Login using Logmein

Group16_QuadMasters:Selective Repeat ARQ

Selective Repeat ARQ is a specific instance of the Protocol, in which the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N ARQ, the receiving process will continue to accept and frames sent after an initial error.

The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every ACK it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window. The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame. Once the sender has sent all the frames in its window, it re-sends the frame number given by the , and then continues where it left off.

When used as the protocol for the delivery of subdivided messages it works somewhat differently. In non-continuous channels where messages may be variable in length, standard ARQ or Hybrid ARQ protocols may treat the message as a single unit. Alternately selective retransmission may be employed in conjunction with the basic ARQ mechanism where the message is first subdivided into sub-blocks (typically of fixed length). The original variable length message is thus represented as a concatenation of a variable number of sub-blocks. While in standard ARQ the message as a whole is either acknowledged (ACKed) or negatively acknowledged (NAKed), in ARQ with selective transmission the NAKed response would additionally carry a bit flag indicating the identity of each sub-block successfully received. In ARQ with selective retransmission of sub-divided messages each retransmission diminishes in length, needing to only contain the sub-blocks that were NAKed. In most channel models with variable length messages, the probability of error-free reception diminishes in inverse proportion with increasing message length. In other words it’s easier to receive a short message than a longer message. Therefore standard ARQ techniques involving variable length messages have increased difficulty delivering longer messages, as each repeat is the full length. Selective retransmission applied to variable length messages completely eliminates the difficulty in delivering longer messages, as successfully delivered sub-blocks are retained after each transmission, and the number of outstanding sub-blocks in following transmissions diminishes.

Group16_QuadMasters:Mandatory Blog

Team Name :QuadMasters

Team Members : Pavan.S[IS64],Hariprasad.N.R[IS30],Deepakraj[IS23],Prakash[IS69]

Group A Project Chosen : Remote Login

Group B Project Chosen :Selective Repeat ARQ

Presentation Date of Group A Project : Completed, gave on September 9 2009

Presentation Date of Group B Project : Not Completed, as on December 9 2009

Group A PPT Uploaded : Yes

Group B PPT Uploaded : No

Group A Project Report Made : No

Group B Project Report Made : No

Group A Project problems-faced-and-solved blog uploaded : Yes

Group B Project problems-faced-and-solved blog uploaded : No